The purpose of this work is to study the role of hydrogen bonds in determining structures of molecular aggregates of small organic molecules. The long range goal of this work is to develop sets of empirical hydrogen- bond rules that are functional group specific, so that knowledge about hydrogen-boding properties of a particular functional group can be used to predict hydrogen-bond modes in many different situations, such as at binding sites on enzymes, or in crystals. Crystallographic data about hydrogen bonds will be taken from the Cambridge Crystallographic Data Base and analyzed using graph set analysis, a method developed in our lab for categorizing hydrogen-bond functional groups. The proposed work involves applying this method to a comprehensive set of common organic functional groups. Empirical hydrogen-bond rules will be derived in part from graph set assignments and will be used as a way to predict likely configurations of hydrogen-bond data since the focus is on complementarity of hydrogen- bonding groups rather than on hydrogen-bond geometry. To make these rules generally useful, it is necessary to know the relative proton-donating and accepting abilities of competitive functional groups in addition to their individual modes of hydrogen-bond formation. It is proposed here to prepare systematic sets of cocrystals containing competitive groups to see which of several possible hydrogen-bond interactions have formed. Standard molecules that are exclusively proton acceptors of proton donors will be used so the hydrogen-bond competitions can be easily controlled. There are many compounds that are exclusively acceptors, but very few that are exclusively donors. One such molecule discovered in our lab, (N, N'-' bis (m-nitrophenyl) urea), is also a very general complexing agent so it will be used as a cocrystallization substrate. The selectivity ratings determined in this manner will be tested by designing and preparing cocrystals of multifunctional molecules. The principles developed here should be generally applicable to predicting hydrogen-bond selectivity at receptor sites, during molecular aggregation in solution, and in crystals.